Among all the qualities contributing to comfort in modern means of transport, such as trains and motor vehicles, silence has become the determining factor.
Acoustic comfort has been improved now over several years, by dealing with noise, such as noise from the engine, tyres or suspension, and this at their source or during their propagation through the air or solids, by means for example of absorbent coatings or elastomeric connecting components.
The shapes of vehicles have also been modified in order to improve penetration through the air and to reduce turbulence that is itself the source of noise.
For several years, emphasis has been given to the role that glazing can play in improving acoustic comfort, in particular laminated glazing having plastic film inserts. Laminated glazing has moreover other advantages such as the elimination of the risk of flying fragments in the case of sudden breakage, so as to delay breakage.
It has been demonstrated that the use of standard plastic films in laminated glazing is not suitable for improving acoustic comfort. Specific plastic films have thus been developed that have damping properties, enabling acoustic comfort to be improved.
In the following description, reference to a damping film relates to a viscoelastic plastic film that provides improved damping of vibrations in order to give glazing the function of noise reduction.
It has been shown that the acoustic performance of glazing depends on the value of the loss factor tan δ of the material constituting the insert film. The loss factor is the ratio between the energy dissipated in the form of heat and the energy of elastic deformation. It characterises the capacity of the material to dissipate energy. The higher the loss factor, the greater the energy dissipated, and therefore the more the material plays its damping role.
This loss factor varies according to temperature and frequency. For a given frequency, the loss factor reaches a maximum value at a temperature called the glass transition temperature.
The materials used as inserts for laminated glazing are viscoelastic plastic films, of the acrylic polymer or acetal resin type for example, which have a quite high loss factor, such that is at least greater than 0.6 for a given temperature range and for a given frequency range.
In order to provide damping properties enabling acoustic comfort to be improved, some patents have revealed the necessity, apart from having a loss factor greater than a threshold value, of preventing a sudden fall in the loss factor at the coincidence frequency (typically at around 2000 Hz) specific to the composition of the laminated glazing. In order to arrive at this, U.S. Pat. No. 5,190,826 shows the association of at least two insert films or the combination of two materials in order to constitute an insert film, such that each of the films or materials consists of a distinct polyvinyl acetyl resin mixed with a plasticizer. This association of two specific types of resin would make it possible to provide damping over a wide temperature range.
U.S. Pat. No. 5,796,055 also describes the combination of two films that have a high damping property (high loss factor tan δ) over two distinct temperature ranges so as to obtain laminated glazing which provides acoustic performance over a wider temperature range.
Although effectively, when two films, exhibiting improved damping properties over two distinct temperature ranges are combined, the insert finally obtained can provide the laminated glazing with high damping over a more extended range of temperatures than the latter taken separately, but on the other hand this combination does not necessarily lead on the one hand to optimized damping over this more extended temperature range and, on the other hand, it is not invariably efficient.